Roofing granules are extensively used in roll roofing and asphalt shingle compositions. Such granules are generally embedded in the asphalt coating on the surface of an asphalt impregnated base material such as roofing felt with the granules thus forming a coating that provides an adherent weather resistant exterior roofing surface. As the outer granule coating also provides the aesthetic effect observable with respect to the roof to which the roofing material is applied, the appearance of the granules is of major marketing importance. For this reason, a pigmented color coat is ordinarily applied to the base mineral granules to enhance their visual decorative effect.
Kaolin clay is used extensively in silicate paint formulations for coloring roofing granules. It serves as a filler, extender, moisture release agent and reactant to aid film insolubilization during high temperature firing. Although clay is a major component of such coating formulations, it alone lacks the brightness and opacity needed to hide the dark underlying base rock of the granule. Although kaolin clay consists mainly of alumino silicates, other constituents are present as a result of the clays natural origin. Iron and titanium impurities, for example, impart a buff or yellow color to the clay while organic impurities such as humic acid derivatives cause a grey coloration observed in sedimentary kaolin. Unbound iron in the form of extraneous Fe (III) minerals also causes discoloration. For these reasons, white colored roofing granule coatings using natural kaolin clay frequently require appreciable amounts of expensive TiO.sub.2 to achieve desired color specifications.
White or light colored roofs are particularly favored in many areas. Titanium dioxide pigment is commonly used in the production of such white or light colored roofing granules. The TiO.sub.2 is commonly used in conventional insolubilized alkali silicate coatings, such as those described in U.S. Pat. Nos. 2,379,358 to Jewett, 3,255,031 to Lodge et al, 3,479,201 to Sloan. As mentioned above, the kaolin clay which is frequently used in such coatings contains impurities such as iron, titanium and humic acid which tend to discolor the clay. Organic impurities, in particular, cause the clay to draken upon exposure to high temperature. This requires the use of larger amounts of TiO.sub.2 then would otherwise be necessary or desirable.
The pigment requirements in silicate-clay coating formulations, particularly TiO.sub.2 in white coatings, can be reduced by increasing the brightness of the clay. This can be achieved by oxidation and/or reduction bleaching to remove discoloring clay impurities. Pigment requirements can be further reduced by increasing the opacity, or hiding power, of the coating itself. This can be accomplished by using clays and pigments of higher purity and smaller particle size. The use of calcined or specially opacified clays can also be effective. Unfortunatley, the use of raw materials of a higher state of purity and/or subdivision generally increases the cost.
The intentional introduction of microvoids in a film to induce opacification is a well-known and inexpensive method of reducing pigment requirements. Voids can, for instance, be added in the form of hollow glass or plastic spheres. Use of such preformed voids eliminates the dependence on drying conditions, nature of the coating vehicle, and other factors governing in-situ void formation. Preformed voids are best suited for coatings that are designed for ambient cure. Voids can also be created in films by incorporating additives that induce cracks and microfractures during film curing. For example, polymer films containing UV light-curable components that shrink during curing are known. As the curable and non-curable components separate, opacifying microfractures appear at the phase boundaries. Light-scattering microvoids can also be formed by the evaporation of a droplet of volatile liquid, which is either emulsified into the coating or formed by precipitation during film formation. Loss of the volatile liquid takes place by diffusion through the coating matrix as the volatile liquid is simultaneously replaced by air.
Voids in a transparent film act as opacifiers by scattering part of the incident light that traverses the air/film interfaces. Sodium silicate films, for example, are highly transparent when dried at ambient temperature, but become increasingly white and opaque as more complete water loss creates voids at higher temperatures. Unfortunately, the voids formed by drying neat silicate films are large and scatter light inefficiently. In addition, the film is weak because the voids are interconnected and the film surface is extensively disrupted. Similarly, premature gelling of incompletely dried silicate films also induces opacification by changing the film structure from a glass to an amorphous coating of individual light-scattering silica particles. This change is analogous to that of the differences between ice and snow. Amorphous coatings, however, are powdery and weak because there is little binding between the particles or to the substrate.